Data protection, used for backing up various types of data, from simple letters to mission-critical enterprise data, has typically been implemented by duplicating data via techniques such as mirroring with RAID (redundant array of independent disks). Mirroring relates to the physical media on which the data is stored, and protects against media failure. When a logical data loss or corruption occurs, such as an accidental file deletion or a virus, the data must be restored from a backup system.
Even when used in conjunction with mirroring, all of the disks in a RAID system reflect the current state of the primary disk; i.e., any data loss or corruption will also exist on the disks in the array. Backup copies are generally made on a periodic basis, and reflect the state of the primary disk at a given point in time. Because backups are not made on a continuous basis, when data is restored, there will be some data loss from the time of the backup until the time of the data loss. In a mission-critical setting, such a data loss, even if only for a brief period, can be catastrophic. Beyond the potential data loss, restoring a primary disk from a backup system, can become complicated and take several hours or more. This additional downtime further exacerbates the problems associated with a logical data loss.
The traditional process of backing up data to tape media and time driven and time dependent. A backup process typically is run at regular intervals and covers a certain period of time. For example, a full system backup may be run once a week on a weekend, and incremental backups may be run every weekday at some time after the close of business (usually overnight). These individual backups are then saved for a predetermined period of time, according to a retention policy. In order to conserve tape media and storage space, older backups are gradually faded out and replaced by newer backups. Further to the above example, after a full weekly backup is completed, the daily incremental backups for the preceding week may be discarded, and each weekly backup may be maintained for a few months, to be replaced by monthly backups.
While the backup creation process can be automated to a great extent, restoring data from a backup remains a manual and time-critical process. First, the appropriate backup tapes need to be located, including the latest full backup and any incremental backups made since the last full backup. In the event that only a partial restoration is required, locating the appropriate backup tape can take even longer. Then after the backup tapes are located, they must be restored to the primary disk. As can be imagined, even under the best of circumstances, this type of backup and restore process cannot guarantee high availability of data.
Another type of data protection involves making point in time (PIT) copies of data. A first type of PIT copy is a hardware-based PIT copy, which is a mirror of the primary disk onto a secondary disk. The main drawbacks to a hardware-based PIT copy are that the data ages quickly and that each copy takes up as much disk space as the primary disk. A software-based PIT, typically called a “snapshot,” is a “picture” of a file system's data structure. Various types of software-based PITs exist, and most are tied to a particular platform, operating system, or file system. These snapshots also have drawbacks, including occupying space on the primary disk, rapid aging, and possible dependencies on data stored on the primary disk wherein data corruption on the primary disk leads to corruption of the snapshot. Furthermore, both types of PIT techniques still require the traditional tape-based backup and restore process.
Using a PIT copy means that a copy is made at a discrete moment in time, and restores based upon that PIT copy are based on that discrete moment in time. In contrast an any point in time (APIT) copy implies all moments in time, i.e., a continuous copy. It is therefore desirable to have a continuous data protection scheme in which a copy can be selected from any point in time.